Watershed boundaries closely surround one area

Question

I am trying to make an average of two blobs in OpenCV. To achieve that I was planning to use watershed algorithm on the image preprocessed in the following way:

cv::Mat common, diff, processed, result;
cv::bitwise_and(blob1, blob2, common); //calc common area of the two blobs
cv::absdiff(blob1, blob2, diff);       //calc area where they differ

cv::distanceTransform(diff, processed, CV_DIST_L2, 3); //idea here is that the highest intensity 
                                                       //will be in the middle of the differing area
cv::normalize(processed, processed, 0, 255, cv::NORM_MINMAX, CV_8U); //convert floats to bytes

cv::Mat watershedMarkers, watershedOutline;
common.convertTo(watershedMarkers, CV_32S, 1. / 255, 1); //change background to label 1, common area to label 2
watershedMarkers.setTo(0, processed); //set 0 (unknown) for area where blobs differ

cv::cvtColor(processed, processed, CV_GRAY2RGB); //watershed wants 3 channels
cv::watershed(processed, watershedMarkers);
cv::rectangle(watershedMarkers, cv::Rect(0, 0, watershedMarkers.cols, watershedMarkers.rows), 1); //remove the outline

//draw the boundary in red (for debugging)
watershedMarkers.convertTo(watershedOutline, CV_16S);
cv::threshold(watershedOutline, watershedOutline, 0, 255, CV_THRESH_BINARY_INV);
watershedOutline.convertTo(watershedOutline, CV_8U);
processed.setTo(cv::Scalar(CV_RGB(255, 0, 0)), watershedOutline);

//convert computed labels back to mask (blob), less relevant but shows my ultimate goal
watershedMarkers.convertTo(watershedMarkers, CV_8U);
cv::threshold(watershedMarkers, watershedMarkers, 1, 0, CV_THRESH_TOZERO_INV);
cv::bitwise_not(watershedMarkers * 255, result);

My problem with the results is that the calculated boundary is (almost) always adjacent to the area common to both blobs. Here are the pictures:

Input markers (black = 0, gray = 1, white = 2)

Watershed input image (distance transform result) with resulting outline drawn in red:

I would expect the boundary to go along the maximum intensity region of the input (that is, along the middle of the differing area). Instead (as you can see) it mostly goes around the area marked as 2, with a bit shifted to touch the background (marked as 1). Do I do something wrong here, or did I misunderstand how watershed works?

Answer 1

Starting from this image:

You can get the correct result simply passing an all-zero image to watershed algorithm. The "basin" is then equally filled of "water" starting from each "side" (then just remember to remove the outer border which is set by default to -1 by watershed algorithm):

Code:

#include <opencv2\opencv.hpp>

using namespace cv;
using namespace std;

int main()
{
    Mat1b img = imread("path_to_image", IMREAD_GRAYSCALE);

    Mat1i markers(img.rows, img.cols, int(0));
    markers.setTo(1, img == 128);
    markers.setTo(2, img == 255);

    Mat3b image(markers.rows, markers.cols, Vec3b(0,0,0));
    markers.convertTo(markers, CV_32S);
    watershed(image, markers);

    Mat3b result;
    cvtColor(img, result, COLOR_GRAY2BGR);
    result.setTo(Scalar(0, 0, 255), markers == -1);

    imshow("Result", result);
    waitKey();

    return(0);
}